Gravel packing of a well borehole involves placing sized gravel or sand within a wellbore external to a screen. The gravel pack sand and screen slots are sized to prevent formation sand migration into the wellbore. Gravel packing is critical to continued oil production from subterranean formations consisting of loosely consolidated or unconsolidated sand. Without an effective gravel pack, the produced sand from the formation will erode equipment and fill tanks and vessels, thus causing the need for shutdowns to clean or replace equipment. Sand can also fill the wellbore and interfere with effective oil lifting. Additionally, displaced sand can leave cavities around the wellbore and thus jeopardize lateral support for well tubulars. Loss of lateral support may result in damage to well tubulars.
The screen is typically hung from a packer and extends downward into a borehole. The borehole can be either cased or uncased. An annulus between the screen and the casing is typically sealed on top and bottom by packers.
Placement of sand around the screen is accomplished by circulating a slurry of sand suspended in a carrier fluid from the surface through a drill string to a crossover tool immediately above the screen. The crossover tool directs the slurry from the drill string to the annulus surrounding the screen. As the slurry travels down the annulus around the screen, some carrier fluid may enter the formation, leaving sand deposited at the perimeter of the wellbore or perforations. The remaining carrier fluid will pass through the screen. The screen has openings that are sized to retain the sand outside of the screen. Sand will therefore be filtered from the slurry by the screen and remain in the annulus outside of the screen. A wash pipe is usually provided within the screen extending from the crossover tool to near the bottom of the screen. Returning carrier fluid, after passing through the screen, enters the wash pipe at the lower end of the wash pipe, and travels up the wash pipe to the crossover tool. The crossover tool directs this returning carrier fluid to the annulus outside of the drill pipe, above the screen packers, and up to the surface.
After the annulus between the screen and the borehole (or the casing if a cased borehole is to be gravel packed) is filled with sand, the crossover tool is disconnected from the screen, and removed from the wellbore. The screen is left suspended from packers, surrounded by gravel pack sand. A production tubing is then run to the inside of the screen, along with artificial lift means, if required.
A crossover tool is provided to the gravel pack assembly as it is installed in the well. The crossover tool is a removable mechanism attached to the gravel pack packer which directs fluid flow while placing the gravel pack slurry. Slurry is pumped into the drill pipe down the wellbore. The slurry is diverted into the screen annulus via the crossover tool once the slurry reaches the gravel pack packer. The slurry is transported down the screen annulus. As this occurs some may enter the formation and some may enter the screen. Since sand cannot pass through the screen, it is deposited in the screen annulus. Carrier fluid that enters the screen may return up a wash pipe which is installed inside the screen. Fluids returning up the wash pipe are diverted into the drill pipe/casing annulus above the gravel pack packer via the crossover tool to return to surface.
An alternative way to provide a gravel pack around a screen is to first fill the borehole with sand, and then sink the screen into the sand. This has been accomplished by providing an auger at the bottom of the screen and along the length of the screen. The screen is then "screwed" into the sand. A substantially studier screen is required, and the depth to which the screen may be placed within the sand is limited.
Another method to sink a screen into sand which has been placed in a borehole is to fluidize the sand from the bottom of the screen by passing fluids through a wash pipe to an outlet at the bottom of the screen. This is known as a "wash down" method.
Methods which sink the screen into previously placed sand require simpler tools because the cross over tool is not required. But, providing a dense sand pack with these prior art methods is not possible. The washdown method fluffs the sand as the screen is lowered, and the auger method lifts sand and can create voids near the screen.
To be effective, the gravel pack must comprise densely packed sand without voids or cavities in the sand. If portions of the annulus around the screen are not packed completely with sand, formation fluids containing formation sand will quickly erode the screen, leading to a gravel pack failure. Further, if the gravel pack initially is not densely packed, subsequent compaction caused by, for example, flow of the formation fluids, can result in voids and cavities within the gravel pack.
Known methods to increase the density of gravel packs include pulsing the flow of the return fluid as disclosed in U.S. Pat. No 3,830,294. Pulsing of the fluid flow is helpful in increasing the density of the gravel pack, but merely pulsing the flow imparts a limited amount of energy into the gravel pack, and can have deleterious effects, such as fracturing the formation. Other methods to pulse flows of drilling fluids have been developed for the purpose of transmitting information to the wellhead. These are described in, for example, U.S. Pat. Nos. 4,291,395, 4,323,991, 4,775,016 and 5,009,272. Like '294, these methods may not impart a significant amount of energy into a gravel pack.
A method to vibrate a drillstring and gravel pack apparatus by imparting a sonic frequency vibration which may be a resonant frequency at the wellhead is disclosed in U.S. Pat. Nos. 4,599,031 and 4,665,980. This method would be useful if the drill pipe did not have significant contact with the wellbore or casing above the gravel pack. Unfortunately, this is rarely the case. Wellbores inherently drill in a corkscrew configuration. A drill pipe is therefore in frequent if not almost continuous contact with the wellbore walls or casing. Vibrations imparted at the wellhead can therefore be significantly dampened, and vibrations of only a small magnitude may be present at the gravel packing apparatus. It would be preferable to impart vibrations at the gravel packing apparatus directly to minimize dampening of the vibrations, and to utilize more efficiently the energy to impart these vibrations.
U.S. Pat. No. 3,113,621 discloses the use of known vibration imparting tools to well liners to add gravel to a wellbore through the liner by vibrating larger sand particles through the liner than can return into the liner without vibration. The method to impart the vibration requires using known electrically driven or hydraulically driven vibrators. Thus, a 10 to 100 horsepower motor, along with a power source, must be inserted into the wellbore It would be preferably to impart such vibration with a more simple, less expensive, and more compact source of vibration.
Bluff objects are known to shed vortexes at rates which can be proportional to the flow rate of fluid passing the bluff object. This phenomena is utilized in flow rate measuring devices disclosed in, for example, U.S. Pat. Nos. 3,535,927, 3,927,566 and 4,026,150. U.S. Pat. No. 3,927,566 further discloses the use of vortex shedding of one bluff object to move a second bluff object up and down. The second bluff object is located immediately downstream of the first bluff object The frequency of the up and down movement of the second bluff object is, according to Birkoff's Theory, proportional to the rate of flow of the fluid past the bluff objects. Although the second bluff object could impact the walls of the flow conduit, such impacting would render the device unreliable as a flow measurement device and thereby defeat the intended purpose of the invention.
It is therefore an object of the present invention to provide a method and an apparatus to provide a dense sand pack surrounding a screen within a wellbore. It is a further object to provide such a method and apparatus wherein sand is deposited within the wellbore before and the screen is then sunk into the sand.